In order to observe the PBHs at high redshift, not only should gravitational wave sensitivity be high but also the event rate of PBH binaries system should be large enough. The estimation of PBH binaries event rate follows [1]}, the condition for decouple of PBH binaries from FRW background at matter-radiation equality \(z = z_{\mathrm {eq}}\)  is
\(x < f^{1/3}_{\mathrm {PBH}} \bar{x}~.\) 
